[FieldTrip] evoked power greater than total power

Russell G Port russgport at gmail.com
Thu Jan 24 23:07:35 CET 2013

So my script generally goes preprocessing:








cfg.outputfile= -------


then from this output I do two different things, first I produced the time
locked data:

[timelock] = ft_timelockanalysis(cfg, data)

For this new variable I then run the ft_freqanalysis with the following



cfg.foi= 5:0.95:100



and so this gives me the evoked power

for the total power instead i take the data from the preprocessing function
(called data above) and repeat the exact same frequency transformation
(same cfg setting/same called function)

Once this is done, I convert both to dB scale using freqbaseline (-.2 to 0)
and perforimg log10*10 operation of the data. Its here where suddenly total
power is less than evoked power. I assume then its the baseline correction
(since the total power has considerably more baseline activity (activity in
the -.2 to 0 time window)) that creates an 'artificially' lower total power
to evoked power.

On Thu, Jan 24, 2013 at 4:45 PM, Rojas, Don <Don.Rojas at ucdenver.edu> wrote:

> Russ,
> It might help if you posted a bit of your script so that we could see how
> you are deriving the measures. But, the relationship you state is true only
> given a couple of assumptions. First it depends on how you define induced
> power. For some, induced power is the total signal, phase-locked and
> non-phase-locked, i.e., the average in the frequency domain. So in that
> sense, induced = total. If you are going with the notion that induced =
> total - evoked, then yes, you would expect that evoked power would always
> necessarily be less than or equal to total power. But second, that
> relationship only holds up if you do not baseline correct the power
> estimates you are comparing, which is often done in time-frequency
> analyses. As far as I recall, in ft_freqanalysis there is no particular
> option to return only induced or evoked power in the way that you mention
> it, but certainly it could be calculated from the output of that function.
> So if you let us know what your cfg options were and subsequent
> calculations, it might be more helpful.
> Best,
> Don
> -----------------------
> Don Rojas, Ph.D.
> UCD Magnetoencephalography Lab
> University of Colorado Denver Anschutz Medical Campus
> 13001 E. 17th Pl F546
> Aurora, CO 80045
> On Jan 24, 2013, at 2:03 PM, Russell G Port wrote:
> > Hi All,
> >
> > I am currently trying to understand something and I think fieldtrip
> people will already know whats going on. I run my data, from both in-vivo
> EEG recordings in animals and MEG in children, through freqanalysis. I have
> the strange result though that my total power (in either mV^2 or nAm^2) is
> less than my evoked power. How can this be, if total=evoked+induced. I am
> assuming that something that I have done while handling the data is causing
> this error, but I have check my work very carefully. Has anyone ever seen
> anything like this before, or can suggest what I am doing to help point me
> in the right direction?
> >
> > Cheers - Russ
> > _______________________________________________
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